Photoelectric vs Compton effect | Visual Explanation

There are two major ways in which  diagnostic x-rays interact with tissue. The first one is the photoelectric effect,  and the second one is the Compton effect. The photoelectric effect is the process of a  photon being absorbed by an inner shell electron.

All the photon energy is transferred to  the electron so that the photon disappears. An energetic photoelectron is then  ejected and this leaves a vacancy. To stabilize the atom, an outer shell  electron fills the vacancy in the inner shell.

Each electron shell has a different energy level,  with those closest to the nucleus being lower in energy than those farther from the nucleus. In  order to move between shells, an electron must absorb or release an amount of energy. This is why  if an electron drops down to a lower-energy shell, it will release energy in the form of x-rays. 

This is also known as characteristic radiation. In Compton scattering, the photon transfers only  a part of its energy to an outer shell electron. The photon knocks the electron out, but unlike  in the photoelectric effect, the scattered photon still has a fraction of the energy of the incoming  photon and can still travel through the patient.

These scattered photons may then proceed  to generate further interactions both inside and outside the patient. This is why  Compton scattering not only decreases the image quality but also increases the patiënt dose and  is the predominant source of background radiation. So In the photoelectric effect, a single electron absorbs all the energy of an incident  photon, while in the Compton effect, the incident photon only transfers a  portion of its energy to an electron.

The likelihood of photoelectric absorption  increases with the atomic number and decreases with increasing photon energy. Therefore, as  the atomic number increases, the photoelectric absorption becomes more pronounced. This is why  materials such as lead (Z=82) are widely used for shielding gamma and x-rays.

The photoelectric  effect is dominant at the lower photon energies, the Compton effect takes over at medium  energies and pair production at higher energies. Soft tissue is made up of low atomic number  elements such as hydrogen, oxygen and carbon while bone is made up of higher atomic number  elements such as calcium and phosphorus. Therefore photoelectric absorption is more likely  to occur in bone than in tissue.

This effect is important in providing contrast. If all x-rays  were to pass through the patient, the image would be uniformly dark and if all the x-rays were to  be absorbed, the image would be uniformly white. Another important concept is that of the ‘k-edge’. 

The k-edge represents the energy needed to eject a K-shell electron. You’ll see an abrupt increase in  the photoelectric absorption of x-ray photons at the energy level just beyond the binding energy of  the k-shell electrons. In other words, at energies just below the k-edge the photons don’t have  enough energy to free the k-shell electrons.

As the energy increases to  just over the required energy, the probability of the photon being attenuated by  a photoelectric reaction significantly increases. Iodine and gadolinium-based compounds are the  preferred contrast agents because they have K-edges around 30-50 keV, which is right  in the middle of the x-ray beam spectrum. This is why tissues with even a small amount  of iodine will have a much stronger x-ray stopping power than those without iodine. 

Also, they both have high atomic numbers and the higher the atomic number, the more  photoelectric absorption that will occur. So in summary, In the practice of nuclear  medicine, where gamma rays between 50 and 550 kilo-electron volt are used, Compton scattering is  the dominant type of interaction in material with lower atomic numbers, such as human tissue and  air. The photoelectric effect is the dominant type of interaction in materials with higher atomic  numbers, such as contrast agents, bone and lead.